Energy & Fuels, Vol.25, No.11, 5366-5377, 2011
Effect of Pressure Drop Due to Grate-Bed Resistance on the Performance of a Downdraft Gasifier
The grate-bed resistance coefficient appears to be an important operating parameter with a strong influence on overall performance during downdraft fixed-bed gasification. It directly affects the velocity profile, temperature distribution, and height of the bed. To date, no information on the pressure drop as a result of the grate-bed resistance has been found. The objective of the present investigation is to propose a correlation that can predict the total effect of pressure drop through a grate of a certain surface porosity covered by the porous bed. The term related to the grate-bed resistance is based on the effective grate porosity, which combines surface bed porosity with geometrical criteria of the grate. On the basis of these criteria, a new term has been integrated into the Ergun equation. The prediction has been validated against the experimental results and conducted on a 0.7 MWth downdraft, fixed-bed gasifier that was fueled with wood pellets of a feeding rate 50-100 kg/h. Three types of grates of different porosities (0.6, 0.2, and 0.04) and thicknesses (2 mm and 10 mm) of an orifice diameter of 6 mm each have been tested under various operating conditions. The oxidizer (air or mixture of air and steam) was preheated over 1000 degrees C and supplied with a rate of 60-110 kg/h. The predicted values are in agreement with the experimental results. Although lower grate porosity, higher conversion of fuel, and heating value of the gas is produced, the stability of the process is disturbed. Therefore, a grate porosity reduction below 20% is not recommended.